1. Field of the Invention
This invention relates to single shaft gas turbine engine vehicle power systems and more particularly to vehicle power systems with automatic engine speed and transmission ratio control.
2. Description of the Prior Art
A turbine engine having a power turbine rotor which is directly coupled for rotation with a compressor rotor may be referred to as a single shaft gas turbine engine. Such engines are used extensively for fixed speed applications such as generation of electricity because of their low initial cost and superior reliability. However, the curve representing torque variations with respect to engine speed for such engines has a steep, narrow peak. As a result, a single shaft gas turbine engine develops maximum torque and power at a particular engine speed which is typically in the range of 50,000 to 70,000 R.P.M. This speed at which maximum torque and power are developed is often referred to as 100% rated speed, and torque and power decrease rapidly as engine speed increases or decreases away from the 100% rated speed.
Because of this torque-speed characteristic single shaft gas turbine engines have not been extensively used in vehicular applications where a substantial and continuous range of operating speeds is required. However, a single shaft gas turbine engine may be utilized to advantage in a vehicular power system when coupled with an infinitely variable transmission. With a proper control system, operation of such an arrangement may be maintained with the transmission ratio controlled to provide engine operation at 100% rated speed under full load conditions. Under part load conditions the transmission ratio may be continuously adjusted for optimum part load fuel consumption irrespective of vehicle speed. An automatic control system for such an arrangement was presented to the Society of Automotive Engineers Mid-Year Meeting, Montreal, Quebec, Canada, June 7-11, 1971. The presentation has been published as SAE publication number 710551, "Controls for Single Shaft Gas Turbine Vehicles," by Bernard E. Poore.
The present invention provides further improvements in power systems of the type therein described. For example, the energy required to start a vehicular gas turbine engine is considerable. A single shaft gas turbine engine must typically be accelerated to about 55% of its rated speed before engine operation becomes self sustaining. At this speed the engine is rotating at several thousand R.P.M. and considerable kinetic energy is possessed by the rotating parts of the engine. The engine starting system must supply not only this kinetic energy, but also energy to overcome engine friction and energy to drive vehicle accessories and their drive gears. Connection of vehicle accessories "behind" the service clutch would mean interruption of the accessories each time the service clutch is disengaged. This would be undesirable and unnecessary for accessories such as air conditioner compressor and perhaps unacceptable for accessories such as an alternator, an hydraulic pump or an air compressor. Furthermore, an automatically controlled turbine power system may be somewhat more subject to engine stall under heavy load conditions where a vehicle operator is anticipating automatic operation and an overload condition causes engine stall before the operator can act to relieve the overload. In the event of a stall a considerable delay is encountered as the engine is restarted, and, as explained above, a considerable burden is placed on the starting system.
Another problem associated with known vehicular turbine power systems is an inadequate indication of vehicle operating conditions. In a conventional gasoline engine or diesel engine vehicle power system, there is a substantial relationship between engine speed and vehicle load. A tachometer indication is thus adequate for proper vehicle operation. However, when a vehicle is powered by a turbine engine and infinitely variable transmission having an automatic control system there may be little relationship between engine speed and vehicle load. Some further indication of vehicle load condition then becomes desirable.